Multistage vacuum pump



March 5, 1968 J. LE-BLANC, JR 3,371,857

MULT'ISTAGE VACUUM PUMP 5 Sheets-Sheet 2 Filed Oct. 24, 1965 h 0 w W A a f MA m E m J A'I'TYS.

MULTIST'AGB VACUUM PUMP Filed Oct. 24, 1965 5 Sheets-$heet 5 INVENTOR JOSEPH A. LEBLANQJR A TTYS.

March 5, 1968 J. A. LE BLANC, JR 3,371,857

MULTISTAGE VACUUM PUMP Filed Oct. 24, 1965 5 Sheets-$heet 4 INV'ENTOR JosEPH ALEHMMJI;

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INv ENTOR JOSEPH A.ILEBLAN,C,IR, by W6%, M J; am

5 Sheets Sheet Ann.

J. A. LE BLANC, JR

MULTIS'I'AGE VACUUM PUMP March 5, 1968 Filed Oct. 24, 1965 United States Patent 3,371,857 MULTISTAGE VACUUM PUMP Joseph A. LeBlanc, In, Chicago, 11]., assignor to Precision Scientific Company, Chicago, 11]., a corporation of Delaware Filed Oct. 24, 1965, Ser. No. 504,942 8 Claims. (Cl. 230-152) ABSTRACT OF THE DISCLOSURE An improved multistage vacuum pump of the internal vane type of substantially any desired capacity may be built from standardized parts by varying the axial dimensions of the rotor, vanes and stator of the intake stage (i.e., the same radial dimensions are employed throughout for all pump capacities). The desired objectives are achieved by employing a pair of pumping units arranged sideby-side with a separator plate imposed between the two units and a pair of end plates disposed at opposite ends of the two units. In place of separate gas passageways through the pump separator plate ordinarily found in multi-stage vacuum pumps, axial passageways extending completely through the rotors and the separator plate are provided to achieve stage-to-stage gas passage. Cavities are provided in the end plates and the separator plate in order to permit transfer of gas from the outlet cavity of the intake stage into the rotor hollows and from the hollows of the rotors to the intake cavity of the exhaust stage. Supplementary outlet cavities are provided in order to permit the exhaust of large volumes of gas during the relatively high pressure start up phases of pump operation.

The present invention relates generally to vacuum pumps. The term multistage vacuum pump will be used hereinafter to refer to a vacuum pump having two or more stages.

It is a primary object of this invention to provide an improved multistage vacuum pump construction which enables a pump of any desired capacity to be built from standardized parts simply by varying the axial dimensions of the intake rotor, vanes and stator, A related object of the invention is to provide such an improved pump construction which enables the same pump height and width, i.e., the same radial dimensions, to be maintained for all pump capacities. It is another object related to the foregoing to provide such a pump construction which reduces the weight and cost of the pump, with such reductions increasing in proportion to the pump capacity.

It is another object of the present invention to provide an improved vacuum pump construction which does not require the gas passageways in the separator plate between the pumping stages to be in register with both the discharge cavity in the stator of the first stage and the inlet cavity in the stator of the second stage, and which also does not require interstage passageways externally of the stators.

A further object of this invention is to provide an improved pump construction of the foregoing type which provides a relatively large conductance between adjacent pumping stages.

It is still another object of this invention to provide such an improved pump construction which prevents the oil which is used to lubricate the pump from backing up into the system that is being evacuated, even when the pump remains connected to the evacuated system after it is turned off. Thus, it is a related object of the invention to provide an improved vacuum pump which cannot contaminate the system being evacuated with lubricating oil.

A still further object of this invention is to provide an 3,371,857 Patented Mar. 5, 1968 improved vacuum pump which runs quietly during normal operation, even in the case of large pump capacities, and does not stall during pump startup. In this connection, it is a more particular object of the invention to provide an improved exhaust stator assembly which provides a relatively small cross section for the gas flow during normal operation, thereby enabling the pump to run quietly, and yet provides a relatively large available volume for the fluid expelled from the pump chambers during start-up.

Yet another object of the present invention is to provide a vacuum pump which has substantially lower power requirements, and thus lower connected load charges, than vacuum pumps proposed heretofore, even while maintaining extremely low pressures.

Still another object of the invention is to provide an improved vacuum pump which achieves improved lubrication with the oil from a conventional oil supply means, and is capable of operating over long periods of time with minimum upkeep and maintenance,

It is another object of this invention to provide an improved vacuum pump which is extremely compact and well balanced. A related object of one aspect of the invention is to provide an improved vane arrangement for a vacuum pump rotor which permits the vanes to travel back and forth over a greater radial distance for any given rotor diameter.

Other objects and advantages of the invention will become apparent upon reading the attached detailed description and by reference to the accompanying drawings in which:

FIGURE 1 is an external side elevation View of a two-stage vacuum pump embodying the present invention;

FIG. 2 is an external elevation view of the left-hand end of the pump shown in FIG. 1;

FIG. 3 is an external elevation view of the right-hand end of the pump shown in FIG. 1;

FIG. 4 is a horizontal section taken along the line 44 in FIG. 1;

FIG. 5 is a vertical section taken along the line 55 in FIG. 1;

FIG. 6 is a vertical section taken along the line 6--6 in FIG. 2;

FIG. 7 is an elevation view of the intake end plate in the pump of FIG. 1;

FIGS. 8 and 9 are fragmentary elevation views of the supplementary exhaust ports and associated valve elements mounted on the end plates in the pump of FIG. 1;

FIG. 10 is an exploded perspective view of the pump of FIG. 1 with the intake stage at the left side and with arrows superimposed thereon to show the paths of gas flow through the pump; and

FIG. 11 is a perspective of one of the sliding vanes used in the rotors of the illustrative pump.

While the invention will be described in connection with a preferred embodiment, it will be understood that it is not intended to limit the invention to the illustrative embodiment but, on the contrary, it is intended to cover the various alternative and equivalent constructions which may be included within the spirit and scope of the appended claims.

Turning now to the drawings, there is shown a twostage vacuum pump 10 constructed in accordance with the present invention and including a housing made up of two cup-shaped sections 11 and 12 which are connected to opposite faces of an annular body member 13 by means of two sets of bolts 14 and 15, respectively. As will be seen from the ensuing discussion, the body member 13 also serves as the stator for the intake stage of the illustrative pump. A conventional gas ballast 16 is mounted at the top of the body member 13.

For the purpose of connecting the pump to the system to be evacuated, an intake port assembly 20 is connected to the body member 13 and opens into an inlet cavity 21 formed in the stator portion of the body member 13. As shown most clearly in FIGS. 4, and 10, the gas entering the intake port is drawn down through an arcuate section 21a of the inlet cavity 21 into a pair of symmetrical lower cavity sections 21b which open into a pump chamber 22 formed by the stator 13 for the intake stage of the pump. As is usual in vacuum pumps, the intake port 20 is connected to the system to be evacuated, such as by means of a rubber hose, and upon driving of the pump by an auxiliary motor, the air or gas is Withdrawn from the system being evacuated and discharged into the atmosphere. In the following discussion, it will be assumed that air is being pumped, but it is to be understood that the term air or gas as used herein and in the appended claims includes any fluid material capable of being pumped.

Turning to the rotor assembly of the pump, a rotor drive shaft 30 extends longitudinally through a bearing sleeve 31 in the rear or discharge housing section 12 and is journaled in a pair of longitudinally spaced end plates 32 and 33 mounted within the housing. A pair of bearing sleeves 32a, 33a are disposed around the shaft 30 within the end plates 32, 33, and a seal indicated at 34 is mounted on the shaft 30 adjacent the bearing sleeve 31 to prevent escape of the sealing and lubricating oil. To keep the shaft 30 from moving endwise within the pump, the shaft is provided with thrust washers 23 which are mounted on opposite sides of a retainer plate 24 which is secured to the outer face of the end plate 33 by means of a plurality of bolts 24a. Similarly, a front retainer plate is secured to the outer face of the front end plate 32 by means of a plurality of bolts 25a. Surrounding the shaft within the intake chamber 22 is a first-stage rotor 35 in the form of a relatively large outer annulus 35a having accurately ground end faces and a smaller inner annulus 35b adapted to receive the shaft 30 and secured thereto by means of a key 36. The outer annulus 35a of the rotor is rigidly connected to the inner annulus 3512 by means of integral ribs 350 which are slotted, along with the outer annulus 35a, to accommodate radially extending vanes 37, 38. In order to bias the vanes 37, 38 outwardly against the stator walls forming the pump chamber 22, a compressed coil spring 39 (FIG. 5) is mounted between the opposed inner ends of the vanes 37, 38 so as to urge the vanes outwardly against the walls of the pump chamber.

As is conventional in vacuum pumps of this type, the shaft 30 and the rotor 35 are eccentric with respect to the pump chamber 22 so that the rotor contacts the stator over a terminal region 41 at the top of the chamber, thereby forming an open crescent between the rotor and the walls of the lower portion of the pump chamber for transferring air from the inlet cavity to the outlet cavity of the stator. As the rotor is driven by a motor connected to an external portion of the shaft 30, the vanes 37, 38 slide radially back and forth as they repeatedly sweep past the inlet cavity 21, drawing air into the crescent, trapping the indrawn air Within the crescent, and finally discharging the trapped air through an outlet cavity 42 formed in the stator 13 at the opposite end of the crescent from the inlet cavity 21.

In accordance with one aspect of this invention, the inner annulus 35b of the rotor is provided with a smaller axial dimension than the outer annulus 35a, and the rotor vanes 37, 38 are U-shaped so that the legs of the U overlap the inner annulus 35b in their innermost or retracted positions. This enables the sliding vanes to travel over a maximum radial distance for any given rotor diameter, so that a relatively small rotor may be used to achieve any given crescent volume. It should be noted that the legs of the U, such as the legs 37a, 37b of the vane 37 shown in FIG. 11, cannot be any longer than the radial thickness of the inner annulus 35b so that the inner ends of the vanes do not abut the drive shaft 30.

The second stage of the illustrative pump is generally similar to the first stage described above. Thus, the second stage pumping unit includes a stator 50 forming an inlet cavity 51 for receiving air from the first pumping stage and passing the same into the second stage pump chamber 52, and an outlet cavity 53 for receiving the air that is discharged from the pump chamber 52. Within the pump chamber 52, a rotor 54 is mounted on the shaft 30 by means of a key 55, this second stage rotor 54 being identical in construction and operation to the firs-t stage rotor 35 described above.

Interposed between the two stages of the illustrative pump is a center separator plate having fiat, finely machined side surfaces facing the opposed side surfaces of the rotors 35 and 54. As will be described in more detail below, the separator plate 60 defines appropriate openings for transferring the air being pumped between the two pumping stages.

In accordance with the present invention, the first stage of the pump is provided with gas transfer means for conducting gas from the pump chamber of the first pumping stage into the interior of the first stage rotor, the separator plate interposed between the two pumping stages forms a gas passageway for conducting gas from the interior of the first stage rotor into the interior of the second stage rotor, and the second pumping stage is provided with gas transfer means for conducting gas from the interior of the second stage rotor into the pump chamber of the second pumping stage. Thus, in the illustrative embodiment the stators 13 and 50, the end plates 32 and 33, the rotors 35 and 54, and the separator plate 60 form continuous interconnecting gas passageways for conducting the air which is expelled from the first pumping chamber 22 through the first stator 13 and the adjacent end plate 32 into the interior of the first hollow rotor 35, then on through the interior of the first rotor and the separator plate 60 into the interior of the second hollow rotor 54, and finally through the interior of the second rotor, the adjacent end plate 33 and the second stator 50 into the second stage pump chamber 52. More particularly, as air is discharged from the first stage pump chamber 22 by means of the rotating vanes 37, 38, the air enters the first stage stator outlet cavity 42 which consists of two arcuate cavity sections 43, 44 interconnected by an axially extending passage 45. This outlet cavity 42 communicates with a gas passageway formed in the inner surface of the adjacent end plate 32, and also with a similar passageway 71 formed in the front or intake side of the separator plate 60. The passageways 70 and 71 are designed to conduct the air downwardly through the end plate 32 and the separator plate 60, respectively, into the interior of the hollow first stage rotor 35. The air then flows through the interior of the rotor 35 and into a plurality of ports 72 which are formed in the separator plate 60 for transferring air between the interiors of both rotors 35 and 54. A pair of small ribs 70a, 71a, are preferably formed in the passageways 70, 71 for preventing the rotor vanes from catching on the edges of the passageways 70, 71. Although the separator plate 60 is shown as including three separate ports 72, it will be understood that any desired number of ports, and of any desired configuration, may be used provided they satisfy the gas conductance requirements of any given pump. Also, it will be appreciated that pump weight considerations enter into the design of the separator plate ports.

From the ports 72, the air being pumped flows axially through the interior of the second stage rotor 54 and then into a passageway 73 formed in the inner surface of the discharge end plate 33, and also into a similar passageway 74 formed in the rear or discharge side of the separator plate 60. These passageways 73, 74 are designed to conduct the air upwardly through the end plate 32 and the separator plate 60, respectively, into the inlet cavity 51 of the second stage stator 50, where the air is drawn into the second pumping unit. As in the case of the passageways 70, 71, the passageways 73, 74 are preferably provided with small ribs 73a, 74a for preventing the edges of the rotor vanes from slipping into the passageways 73, 74.

It will be appreciated that this invention utilizes the relatively large available volume within the hollow rotors to provide a large gas conductance between the two pumping stages. Furthermore, since the gas being pumped enters the second stage pumping unit from the interior of the first stage rotor, rather than from the outlet cavity of the first stage stator as in previous pumps, the size of the openings required in the center separator plate depends solely on the required gas conductance. In vacuum pumps of the prior art, the separator plate openings had to be designed not only to provide the required fluid conductance, but also to achieve fluid communication with both the outlet cavity of the first stage stator and the inlet cavity of the second stage stator; consequently, it was necessary to increase the height and/ or width of the separator plate with increasing pump capacities. In an alternative construction of the prior art, the flow of fluids between stages was carried out externally of the separator plate, which was even bulkier and more impractical than increasing the size of the separator plate.

'By using the interiors of the hollow rotors for the interstage gas flow in accordance with the teachings of this invention, practically any desired pump capacity can be achieved by simply varying the axial dimension of the first stage stator and rotor. Thus, all other parts of the pump may be standardized for all pump capacities and, moreover, even the cross sectional configuration of the intake stage stator and rotor may be standardized so that only the axial dimension of the intake stator, rotor and rotor vanes need be varied to achieve any desired pump capacity. Of course, the pumping ratio between the two stages will vary as the axial dimension of the first stage pumping unit is adjusted to achieve different capacities, but changing the pumping ratio is obviously more desirable from a manufacturing standpoint than changing the dimensions of the second stage pumping unit. The net result is that the same pump height and width, i.e., the same radial dimensions, can be maintained for all pump capacity, thereby substantially reducing the weight and cost of the pump with the reductions increasing with proportion to the pump capacity. Indeed, the economical advantages of this aspect of the invention are so substantial that the larger capacity pumps can be manufactured for only a fraction of the cost of prior art pumps of corresponding capacity.

In order to permit the same cross sectional configuration of the various gas passageways to be maintained over a wide range of pump capacities, supplementary exhaust ports and associated valve means are provided in the two end plates 32, 33 which open to discharge large volumes of air during pump start up, and then close with the decreasing gas pressure until the punm is operating at the normal low pressures. Thus, a pair of ports 80 is provided in the first stage end plate 32 in communication with the outlet cavity 42 of the first stator, and a similar pair of ports 81 is provided in the discharge end plate 33 in communication with the inlet cavity 51 of the second stage stator 50. On the outside surfaces of the two end plates, each of the ports 80, 81 terminates in a conventional flutter valve 82, 83, respectively. As can be seen in FIGURES 8 and 9, each of the flutter valves 82, 83 comprises a small plate 84 of fiat spring steel which is rigidly secured at one end to the end plate 32 or 33, such as by means of screws 85. These valves 82, 83 normally remain closed so that there is no discharge of air through the corresponding ports 80, 8 1. During pump startup, however, the volume of air evacuated by the pump is so large that the air pressure within the ports 81 increases sufiiciently to force the valves 82, 83 open for a brief interval, thereby discharging the initial large volumes of air out into adjacent exhaust cavities formed by the cup-shaped end plate retainers 24 and 25. After this initial burst of air discharge: during start up, the valves 82, 83 close and remain closed during normal pump operation when the substantially smaller amounts of air evacuated by the pump are discharged through the normal exhaust ports. These supplementary exhaust ports 80, 81 are most useful at relatively high pump ratios, such as at a ratio of 5:1 when the amount of air being pumped is five times that which can be exhausted through the conventional exhaust means to be described below. In the case of the smaller pumps, operating at a 1:1 ratio for example, the supplementary ports 80, 81 are useful mainly when there is an excessive build-up of oil leading to correspondingly excessive fluid pressures.

For the purpose of exhausting the air which is discharged through the ports 80, 81, and at the same time limiting the opening movement of the flutter valves 82, 83, the two end plate retainers 24, 25 are designed to form a pair of exhaust cavities 86, 87 which are open at the top of the end plate retainers, as can be seen most clearly in FIG. 10, so that any air discharged through the ports 80, 81 is conducted upwardly and expelled through an outlet port 88 in the same manner as the air exhausted through the conventional exhaust port to be described below. As can be seen in FIG. 6, the exhaust cavities 86, 87 are sufficiently narrow in the axial direction to limit the opening movement of the flutter valve plates 84 so as to prevent excessive fiuxing of the same during the initial air discharge. When the pump is turned off, oil flows downwardly through the open tops of the cavities 86, 87 and trickles through the flutter valves 82, 83 which are slightly open due to relaxation of the spring plates 84 while the pump is shut off. In this connection, the exhaust cavities 86', 87 are designed to extend below the bottom edges of the ports 80, 81 so that the oil flow through these ports continues only until the oil level within the cavities '86, 87 is reduced to the level of the lower extremities of the ports.

In accordance with another aspect of this invention, the exhaust stator assembly of the pump is also provided with a dual exhaust system including supplementary exhaust ports which open during pump start up, when the gas pressures within the pump are at the atmospheric level, and during intermediate pressures in the millimeter range, and then close during normal pump operation when gas pressures are usually in the micron range so that the normal gas discharge is through the conventional exhaust port. This dual exhaust system provides a relatively large available volume for the fluid discharge during pump start up and intermediate operation, and then reduces the available volume during normal operating pressures so that the pump runs quietly during normal operation. Thus, the exhaust stator 50 of the illustrative pump is provided with an outlet cavity 53 which includes a first pair of cavity sections 90 of relatively small volume which communicate with the exhaust stage pump chamber 52 and with an interconnecting axial passageway for conducting air discharged from the chamber 52 into the conventional exhaust port 92. In order to provide a relatively large available volume for the increased air flow during pump start up and intermediate pressures, a second pair of cavity sections 93 of substantially larger volume than the sections 90 also communicate with the pump chamber 52 and with a pair of supplementary exhaust ports 94 formed in the exhaust end plate 33. For the purpose of exhausting gas from the large-volume sections 93 whenever the gas pressure is above a predetermined level, the supplementary exhaust ports 94 cooperate with a flutter valve 95 mounted on the outside surface of the end plate 33 within the exhaust cavity 86. Whenever the gas pressure within the exhaust stator outlet cavity 53 exceeds a predetermined level, the flutter valve 95 opens so that fluid can be exhausted through the dual ports 94 as well as through the conventional exhaust port 92. As soon as the gas pressure drops into the normal operating range, usually in the micron range, the flutter valve 95 closes so that any subsequent gas discharge is through the conventional exhaust port 92 alone. Consequently, it can be seen that the dual exhaust system provides a relatively large available volume for the rel atively high pressures encountered during pump start up and intermediate operation. Then during normal operating pressures, a predetermined portion of the exhaust systemis closed so that the entire exhaust gas flow is through the conventional exhaust port so that the noise level can be maintained at a minimum. As a matter of fact, the present pump design produces only slightly more noise than the motor which drives it, even in the case of pumps of relatively large capacity, so that a large number of pumps may be operated simultaneously in the same operating area without raising the noise level to an objectionable degree.

To permit oil flow past the outer ends of the rotor vanes, a shallow relief 96 is formed in the inner surface of the stator 50 between the small volume cavity sections 90 and the large volume sections 93. A similar relief is provided in the outlet cavity configuration in the intake stator 13.

For the purpose of lubricating and sealing the movable parts of the inventive pump, lubricating oil is fed into the pump through the conventional exhaust port 92 formed in the discharge stator 50. To control the flow of oil through the port 92, a flutter valve plate 101 and a flutter valve frame 102 having an opening 102' formed therein are mounted over the top opening of the port on the upper surface of the stator 50. In order to form a tight oil seal around the periphery of the port 92, the flutter valve plate 101 completely covers the opening 102 as well as a substantial portion of the valve frame 102 surrounding the port, with a mounting screw 103 holding the valve plate 101 and the valve frame 102 in tight bearing engagement with the land 102. To permit the controlled passage of lubricating and sealing oil through the valve plate 101, a slit 104 is formed in the free end of the valve plate so that the opposed edges formed by the slit define an elongated narrow opening which starts at the free end of the plate and extends diametrically across the ports. The slit 104 continues a short distance beyond the port toward the stationary end of the valve plate anchored by the screw 103 and terminates in a release slot provided to prevent further splitting of the valve plate during operation thereof. It will be appreciated that the check valve formed by the split plate 101 is continuously open to permit a continuous flow of oil therethrough, but the size of the opening (and thus the oil flow rate) may be varied by deflecting the opposed edges formed by the slit 104 into or away from the port 92. In actual operation of the pump, these deflections are controlled by the pressure differential across the plate 101. A check valve of this type is described in more detail in copending application Ser. No. 486,808, filed Sept. 13, 1965, entitled, Check Valve for a Vacuum Pump, now U.S. Patent No. 3,326,456, granted June 20, 1967, which is assigned to the assignee of the present invention.

One of the important advantages of this invention is that the large gas conductance provided by the interiors of the hollow rotors also prevents the lubricating oil from backing up into the system that is being evacuated, even when the pump is maintained under vacuum conditions while it is shut olf. It often happens that a vacuum pump may be turned off without taking the precaution of closing a cock between the pump and the system which has been evacuated. Under such conditions, the oil is sucked reversely through the pump (following the same path as the pumped gas but in the reverse direction), and may be sucked right into the evacuating system thereby causing undesirable contamination. In the pump of the present invention, however, the large available volume within the gas flow paths followed by the lubricating oil effectively prevents any drawback of oil into the evacuated system when the pump is turned off but left connected to the system.

Referring particularly to FIG. 6, it can be seen that the end plate retainers 24, 25 are designed to form a pair of cavities 110, 111 which surround the pump shaft 30. When the pump is turned off, a certain quantity of the lubricating and sealing oil flows into these cavities 110, 111 through a pair of axial ports 112, 113 to provide lubrication around the respectively adjacent portions of the shaft 30. Then when the pump is started again, any oil and air which have accumulated in and around the cavities 110, 111 is expelled through the ports 112, 113 into the interiors of the rotors and eventually out through the conventional exhaust port 92.

In order to assemble the various parts of the inventive pump in proper orientation with respect to each other, the center plate 60, the second stage rotor-stator assembly, and the discharge end plate assembly 33 are all bolted to the first stage stator and body member 13 by means of a single pair of bolts 114 and 115. Thus, the top bolt 114 is passed through a plurality of registering bolt slots formed in the upper surfaces of the end plate 33, the stator 50, and the center plate 60 and threaded into a corresponding threaded hole in the body member 13. Similarly, the lower bolt 115 is passed through a plurality of registering bolt slots formed in the lower surfaces of the same elements and threaded into a corresponding hole in the body member 13. The intake end plate 32 is secured to the opposite face of the body member 13 in a similar manner by means of a pair of bolts 116, 117 resting in upper and lower bolt slots formed in the end plate assembly and threaded into corresponding holes in the body member 13. In addition, a plurality of through bolts 118 are received in registering axial slots along both sides of the assembled end plate assemblies, rotor-stator assemblies, and separator plate to firmly interconnect the various units and hold them in properly oriented positions. In order to enclose the aforedescribed assembly in the pump housing, the two housing sections 11 and 12 are moved into abutting relationship with opposite faces of the body member 13 and secured thereto by means of a plurality of bolts as already described above.

As can be seen from the foregoing detailed description, this invention provides an improved multi-stage vacuum pump construction which enables a pump of any desired capacity to be built from standardized parts simply by varying the axial dimensions of the intake rotor and stator. Thus, the improved pump design enables the same pump height and width, i.e., the same radial dimensions, to be maintained for a wide range of pump capacities with attendant reductions in the weight and cost of the pump. Moreover, these reductions increase in proportion to increasing pump capacities. Furthermore, the improved vacuum pump provided by this invention does not require the gas passageways in the separator plate between the pumping stages to be in register with both the discharge cavity in the stator of the first stage and the inlet cavity in the stator of the second stage, and does not require interstage passageways externally of the pump stators. Use of the hollow rotors for gas passageways provides a large gas conductance between adjacent pumping stages and prevents lubricating oil from being drawn back into the system that has been evacuated, even when the pump is left connected to the system when it is turned off. The dual exhaust stator assembly provided by the invention permits the pump to run quietly during normal operation, even in the case of large capacity pumps, and yet does not cause the pump to stall during start up. Because of the excellent balance and the improved gas conductance between pumping stages, the inventive pump also has relatively low power requirements. Finally, the improved vane arrangement in the inventive pump rotor 9 permits the vanes to travel back and forth over a greater radial distance for any rotor diameter, thereby providing a more compact and efficient pump construction.

It will be recognized that fluid transfer means other than those shown in the illustrative embodiment may be provided for conducting gas expelled from the pump chamber of the first pumping stage into the interior of the first-stage rotor, and also from the interior of the secondstage rotor into the pump chamber of the second stage. For example, the gas being pumped could be expelled from the crescent of the first pumping stage directly into the adjacent end plate and/or separator plate and then on into the interior of the first rotor. Similarly, the gas being pumped could be conducted from the interior of the second rotor through appropriate passageways in the adjacent end plate and/or separator plate into the intake end of the second-stage crescent. It will also be appreciated that the various interconnecting gas passageways can be designed with different configurations without departing from this invention. For example, the interior design of the rotors can be readily modified and, if desired, the rotors can even be closed at one end as long as means are provided for conducting gas between each pumping chamber and the separator plate via the rotor interiors.

I claim as my invention:

1. In a vacuum pump, the combination comprising a pair of pumping units arranged side-by-side with a separ'ator plate interposed between the two units and a pair of end plates disposed at opposite ends of the two units, each of said pumping units having a stator and a hollow rotor with the interior of each rotor being open at both ends so as to provide an axial passageway through each rotor, the stator of the first pumping unit and the adjacent end plate and separator plate forming interconnecting passageways for conducting gas discharged from the first pumping unit into the interior of the first rotor, said separator plate forming an axial port for conducting the gas from the interior of the first rotor into the interior of the second rotor, and the stator of the second pumping unit and the adjacent end plate and. separator plate forming interconnecting passageways for conducting gas from the interior of the second rotor into the second pumping unit whereby a relatively large interstage gas conductance is provided by the rotor interiors.

2. In a vacuum pump, the combination comprising a pair of pumping units arranged side-by-side with a separ'ator plate interposed between the two units and a pair of end plates disposed at opposite ends of the two units, each of said pumping units having a stator and a hollow rotor with the interior of each rotor being open at both ends so as to provide an axial passageway through each rotor, said stators, end plates, rotors, and separator plate forming continuous interconnecting gas passageways for conducting gas discharged from the first pumping unit through the first stator and the adjacent end plate and separator plate into the interior of the first rotor, through the interior of the first rotor and the separator plate into the interior of the second rotor, and then through the interior of the second rotor, the adjacent end plate and separator plate, and the second stator into the second pumping unit whereby a relatively large interstage gas conductance is provided by the rotor interiors.

3. In a vacuum pump, the combination comprising a pair of pumping units arranged side-by-side with each unit including a stator forming a pump chamber, an inlet cavity communicating with the pump chamber for introducing gas into the pump chamber, and an outlet cavity communicating with the pump chamber for receiving gas discharged from the pump chamber, and a hollow rotor forming an interior cavity which is open at both ends so as to provide a continuous axial passageway through each rotor, a first end plate disposed at the outside end of the first pumping unit and forming a gas passageway for conducting gas from the outlet cavity of the first stator into the adjacent open end of the interior of the first rotor,

a separator plate interposed between the two pumping units and forming a gas passageway for conducting gas from the interior of one rotor into the interior of the other rotor, and a second end plate disposed at the outside end of the second pumping unit and forming a gas passageway for conducting gas from the interior of the second rotor into the inlet cavity of the second stator whereby a relatively large interstage gas conductance is provided by the interiors of the hollow rotors.

4. In a vacuum pump, the combination comprising a plurality of pumping units arranged side-by-side with each unit including a stator forming a pump chamber, an inlet cavity communicating with the pump chamber for introducing gas into the pump chamber, and an outlet cavity communicating with the pump chamber for receiving gas discharged from the pump chamber, and a hollow rotor forming an interior cavity which is open at both ends so as to provide a continuous axial passageway through each rotor, a separator plate interposed between each adjacent pair of pumping units and forming a gas passageway for conducting gas from the interior of the rotor on one side of the separator plate into the interior of the rotor on the other side of said plate, a first end plate disposed at the outside end of the first pumping unit with said first end plate and said separator plate forming gas passageways for conducting gas from the outlet cavity of the first stator into the interior of the first rotor, and a second end plate disposed at the outside end of the last pumping unit with said second end plate and said separator plate forming gas passageways for conducting gas from the interior of the last rotor into the inlet cavity of the last stator whereby a relatively large interstage gas conductance is provided by the interiors of the hollow rotors.

5. In a vacuum pump, the combination comprising a pair of pumping units arranged side-by-side with a separator plate interposed between the two units and a pair of end plates disposed at opposite ends of the two units, each of said pumping units having a stator and a hollow rotor with the interior of each rotor being open at both ends so as to provide an axial passageway through each rotor, the stator of the first pumping unit and the adjacent end plate forming a first pair of interconnecting passageways for conducting gas discharged from the first pumping unit into the interior of the first rotor, the stator of the first pumping unit and said separator plate forming a second pair of interconnecting passageways for conducting gas discharged from the first pumping unit into the interior of the first rotor, said separator plate also forming an axial port for conducting the gas from the interior of the first rotor into the interior of the second rotor, the stator of the second pumping unit and the adjacent end plate forming interconnecting passageways for conducting gas from the interior of the second rotor into the second pumping unit, and the stator of the second pumping unit and said separator plate forming a second pair of interconnecting passageways for conducting gas from the interior of the second rotor into the second pumping unit whereby a relatively large interstage gas conductance is provided by the interiors of the hollow rotors.

6. In a vacuum pump, the combination comprising a pair of pumping units arranged side-by-side with each pumping unit including a stator forming a pump chamber, a hollow rotor mounted for rotation within the pump chamber, means for admitting gas into the pump chamber and means for receiving gas expelled from the pump chamber by said rotor, fluid transfer means associated with the first pumping unit for conducting gas expelled from the first pump chamber into the interior of the first rotor, a separator plate interposed between the two pumping units and forming a gas passageway for conducting gas from the interior of the rotor of the first pumping unit into the interior of the rotor of the second pumping unit, and fluid transfer means associated with the second pumping unit for conducting gas from the interior of the 1 1 second rotor into the pump chamber of the second pumping unit whereby a relatively large interstage gas conductance is achieved by use of the interiors of the hollow rotors as interconnecting gas passageways.

7. In a vacuum pump, the combination comprising a pair of pumping units arranged side-by-side with each unit including a stator forming a pump chamber, an inlet cavity communicating with the pump chamber for introducing gas into the pump chamber, and an outlet cavity communicating with the pump chamber for receiving gas discharged from the pump cham-ber, and a hollow rotor forming an interior cavity which is open at both ends so as to provide a continuous axial passageway through each rotor, the inlet and outlet cavities of each stator being angularly spaced from each other and in substantial axial alinement with the inlet and outlet cavities, respectively, of the other stator, a separator plate interposed between the two pumping units and forming an interstage port communicating with the interiors of both hollow rotors, and means for conducting gas from the outlet cavity of the first stator to the inlet cavity of the second stator via the interior of the first rotor, the port in said separator plate, and the interior of the second rotor in that order, whereby a relatively large interstage gas conductance is provided by the interiors of the hollow rotors.

8. A method of transferring gas between successive stages of a multi-stage vacuum pump in which each stage includes a stator forming a pump chamber, a hollow rotor mounted for rotation within the pump chamber, means for admitting gas into the pump chamber, means for receivinggas expelled from the pump chamber, and an apertured separator plate disposed between the two pump stages, which method comprises conducting gas expelled from the pump chamber of the first pumping stage into the interior of the hollow rotor of the first stage, then through the separator plate into the interior of the rotor of the next pumping stage, and finally from the interior of the second rotor into the pump chamber of the second stage whereby a relatively large interstage gas conductance is achieved by use of the interior of the hollow rotors as interconnecting gas passageways.

References Cited UNITED STATES PATENTS 160,974 3/1875 Upham 103-136 973,190 10/1910 Green 230-158 994,311 6/1911 Green 230-158 1,505,982 8/1924 Traudt 103-136 1,890,614 12/1932 Klopsteg 230-158 1,988,213 1/1935 Ott 230-138 2,394,166 2/1946 Gibson 230-138 3,237,851 3/1966 LeBlanc 230-153 FOREIGN PATENTS 509,247 7/ 1939 Great Britain.

ROBERT M. WALKER, Primary Examiner.

WILBUR J. GOODLIN, Examiner. 

